This invention relates in general to optical communication systems, and in particular to a system and method for controlling optical amplifier pumps.
Optical communication networks, in particular long-haul networks of lengths greater than 600 kilometers, inevitably suffer from signal attenuation due to variety of factors including scattering, absorption, and bending. To compensate for losses, optical amplifiers are typically placed at regular intervals, e.g. about every 50 kilometers, along the optical transmission path.
Optical amplifiers include rare earth doped fiber amplifiers such as erbium doped fiber amplifiers (EDFAs), Raman amplifiers, and hybrid Raman/EDFA amplifiers. An EDFA operates by passing an optical signal through an erbium-doped fiber segment, and xe2x80x9cpumpingxe2x80x9d the segment with light from another source such as a laser. The pump source excites erbium atoms in the doped segment, which then serves to amplify the optical signal passing through.
In contrast, Raman amplification is more distributed and occurs throughout an optical transmission fiber when it is pumped at an appropriate wavelength or wavelengths. Each Raman amplifier may contain one or more pumps. Gain is achieved over a spectrum of wavelengths longer than the pump wavelength through the process of Stimulated Raman Scattering. The difference between the Raman amplifier pump wavelength and the peak of the associated amplified wavelength spectrum at the longer wavelength is referred to as a xe2x80x9cStokes shift.xe2x80x9d The Stokes shift for a typical silica fiber is approximately 13 THz.
Hybrid Raman/EDFA amplifiers generally combine the qualities of EDFAs and Raman amplifiers. For example, an EDFA pumped at 980 nm may provide suitable amplification in the conventional C-band of wavelengths from about 1525 nm to about 1565 nm. Combining this EDFA with a Raman amplifier pumped at a longer wavelength of, for example 1495 nm, can provide an effective hybrid Raman/EDFA amplifier that provides gain over a wide continuous 80 nm range of wavelengths from about 1525 nm to 1605 nm. This range encompasses both the C-band (from about 1525 nm to about 1565 nm) and L-band (from about 1565 nm to about 1605 nm).
Such optical amplifiers are components in optical communication systems, especially long-haul networks that utilize wavelength division multiplexed (WDM) or dense wavelength division multiplexed (DWDM) signals. Such WDM and DWDM systems transmit a plurality of information channels, e.g. 256 channels over a range of wavelengths.
When optical amplifiers and other components of an optical communication system are functioning properly there should be little, if any, variation of the signal power spectrum over the range of transmitted wavelengths from the nominal value as measured at the receiver. This nominal power spectrum corresponds to the path average intensity (PAI) that should ideally be equal or consistently flat over a range of transmitted wavelengths. Many problems, including optical amplifier pump power degradation or failure, can lead to unwanted deviations in PAI and associated signal-to-noise ratio (SNR) as measured at the receiver. Such deviations in PAI and SNR negatively affect signal detection and system reliability leading to system performance penalties.
Accordingly, there is a need for a system and method that can detect such deviations in PAI and provide for PAI recovery with minimal impact to the SNR by adjusting optical amplifier pump parameters.
An optical communication system consistent with the invention includes a transmitter configured to transmit a plurality of optical signals, and a plurality of optical amplifiers for amplifying the optical signals. Each of the amplifiers includes at least one associated pump with at least one adjustable pump parameter. A path average intensity detector is configured to detect a path average intensity for the signals. In response to the detected path average intensity, a pump parameter controller dynamically adjusts at least one of the pump parameters of at least one of the amplifiers to achieve a desired path average intensity.
A method of controlling path average intensity for a range of transmitted wavelengths in an optical communication system comprising a plurality of optical amplifiers consistent with the invention includes: providing a detector for detecting a deviation in path average intensity; and providing a controller for adjusting at least one optical amplifier pump parameter of at least one of said optical amplifiers in response to the deviation to achieve a desired path average intensity.